1. Field of the Invention
The present invention relates to a critical state estimation system for secondary cells, and a critical state estimation method for secondary cells.
In particular, the invention relates to a critical state estimation system and critical state estimation method which are each adapted for estimation of a “critical state” in terms of a “boundary of a permissible range” of a performance (e.g. current or power input/output performance) of a secondary cell, under a prescribed working condition of the secondary cell.
The “critical state” to be estimated may be:
“inputtable current (Iin)” or “outputtable current (Iout)” that defines a limiting boundary of a permissible range for current to be conducted to or from the secondary cell; or
“inputtable power (Pin)” or “outputtable power (Pout)” that defines a limiting boundary of a permissible range for power to be input (or charged) to, or output (or discharged) from, the secondary cell.
2. Description of the Related Art
A power estimator for a battery has been disclosed in Japanese Patent Application Laid-Open Publication No. 9-171063 (hereafter called “JPA'063”).
In this estimator, based on measurements of a supply current I and a terminal voltage V of the battery, a discharge characteristic of the battery is defined as a relationship between current I and voltage V, by an expression of straight line, such that V=R×I+V0 . . . (exp-1: exp=expression),
where, R: an internal resistance of the battery, which is calculated as a slope of the straight line of I-V relationship, and
Vo: an open-circuit voltage, i.e., the terminal voltage V with the current I cut off to a zero, that represents an emf (electromotive force) of the battery.
The open-circuit voltage Vo is calculated as a V-intercept of the I-V straight line. Then, based on the current I and a temperature T of the battery, a permissible minimum voltage Vmin for a guaranteed service life of the battery is calculated, which is substituted for the voltage V in the expression of I-V straight line to obtain a maximum Imax of the current I.
Then, outputtable power P of the battery is estimated, such thatP=Vmin×Imax  (exp-2).